Field of the Disclosure
Embodiments of the present invention relate to an organic light emitting display device, and more particularly, to an organic light emitting display device which is capable of reducing defects caused by a characteristic variation of a pixel and improving reliability of external compensation by reducing a sensing error, and to a method for driving the same.
Discussion of the Related Art
Recently, there is an increased interest for an organic light emitting display device owing to various advantages. In contrast to a liquid crystal display (LCD) device, the organic light emitting display device can achieve wider viewing angle, and better brightness and contrast ratio. In addition, the organic light emitting display device can emit light in itself, that is, the organic light emitting display device needs no additional backlight unit. Thus, the organic light emitting display device may be manufactured in a thin profile with a light weight, and the organic light emitting display device may have advantages of low power consumption and rapid response speed.
The pixel characteristics of the organic light emitting display device vary depending on driving time and temperature. According to a position of a compensation circuit so as to compensate for the change of the pixel characteristics, there may be an external or internal compensation method. In case of the internal compensation method, the compensation circuit is positioned inside the pixel. Meanwhile, in case of the external compensation method, the compensation circuit is positioned outside the pixel.
Due to the deviations in a process of manufacturing a thin film transistor (TFT) substrate, mobility (k) and threshold voltage of a driving TFT (DT) may vary between pixels. Accordingly, even though a data voltage (Vdata) is identically applied to the driving TFT for each of the pixels in the organic light emitting display device according to the related art, it is difficult to realize uniform picture quality due to a deviation in the electric current flowing in organic light emitting diodes (OLEDs).
In order to overcome this problem, the change in mobility (k) and threshold voltage (Vth) of driving TFT for each pixel may be sensed, and then be compensated so that a driving voltage (k*Vdata+Vth), obtained by adding a compensation voltage (Vth, k) to the data voltage (Vdata) in accordance with a video signal, may be supplied to a gate of the driving TFT.
FIGS. 1 and 2 illustrate a method for sensing the pixel characteristics for the external compensation in the organic light emitting display device according to the related art.
Referring to FIGS. 1 and 2, a method for measuring the characteristics of an OLED panel in the organic light emitting display device may be largely classified into an applied-voltage-based current measuring method and an applied-voltage-based voltage measuring method. The applied-voltage-based voltage measuring method is widely used owing to a shorter measuring time in comparison to that of the applied-voltage-based current measuring method. These methods are based on charging a source terminal of the driving TFT.
In case of the applied-voltage-based voltage measuring method as shown in <S1> of FIG. 1, a voltage is applied to the gate of a driving TFT (Tr3). A current flowing in a source terminal of the driving TFT is charged in a line cap. Thereafter, as shown in <S2> of FIG. 1, a voltage charged by turning off a switching transistor ‘Tr1’ is measured by an analog-to-digital converter (ADC) provided in the display device, thereby sensing the characteristics of the driving TFT.
On assumption that a current change according to a drain-source voltage (Vds) of the driving TFT is identical in a saturation area, the method for sensing the pixel characteristics according to the related art is carried out by charging a source voltage of the driving TFT, and measuring the charged value. The source voltage of the driving TFT is a voltage at the source terminal of the driving TFT, and a drain voltage of the driving TFT is a voltage at the drain terminal of the driving TFT. The drain-source voltage (Vds) of the driving TFT is a voltage across the source and drain terminals of the driving TFT.
However, in reality, a change in the amount of current flowing in the driving TFT varies depending on a variation of the drain-source voltage (Vds) by a modulation effect of a channel, which might cause incorrectness in the amount of electric current measured by the applied-voltage-based voltage measuring method according to the related art.
Also, if the source voltage of driving TFT is increased, the drain-source voltage (Vds) of the driving TFT is decreased in the related art due to the way that the drain voltage of the driving TFT is driven. As such, it is difficult to precisely sense the characteristics of the OLED panel due to the varying characteristics of the driving TFTs.
In case of the applied-voltage-based current measuring method according to the related art, it is assumed that the drain of the driving TFT is fixed to a high power line (Vdd), and the driving TFT is a constant current source. In this case, if the source terminal of the driving TFT is in high-z (high-z) state, a capacitor is fully charged by the current flowing in the driving TFT, whereby the source voltage of the driving TFT is increased.
As shown in FIG. 2 (see S2 area), by measuring the current at the source terminal of the driving TFT twice at times T1 and T2 (sampling times), it is possible to calculate the amount of current (iTFT) flowing in the driving TFT by the following Math Formula 1:iTFT=C*(V2−V1)/Δt  [Math Formula 1]where C is capacitance of storage capacitor, V2 and V1 are voltages at the source terminal of the driving TFT sensed respectively at T1 and T1, and Δt equals T2 minus T1.
FIG. 3 illustrates a change in the constant current (Id) in accordance with the drain-source voltage (Vds) of the driving TFT according to the related art.
Generally, if the source voltage of the driving TFT is increased, the drain-source voltage (Vds) of the driving TFT is decreased proportionally, assuming that the driving TFT is a constant current source. However, in reality, as the drain-source voltage (Vds) of the driving TFT is decreased, the current (Id) of the driving TFT does not remain constant but is also decreased even in the saturation area as shown in FIG. 3. That is, the driving TFT is not driven as the constant current source.
That is, unlike the theory, the current (Id) of the saturation area for the driving TFT as shown in FIG. 3 is changed in accordance with the small change of the drain-source voltage (Vds), and the current (Id) is more sensitive to the change in the drain-source voltage (Vds) when the drain-source voltage (Vds) is equal to or greater than 7V. According to the increase in the source voltage of the driving TFT, a level of the drain-source voltage (Vds) of the driving TFT decreases when the fixed drain voltage is applied. In this case, the amount of current flowing for the driving TFT also varies so that it is difficult to correctly measure the amount of current flowing in the driving TFT.